R. Ishigami

Effects of foreign metals on the thermal stability of noble metal adsorbates at the Si(111) surface

Abstract Effects of monolayer foreign metals on the thermal stability of noble metal adsorbates (Au, Ag, and Cu) at the Si(111) surface have been studied by means of LEED-AES-RBS techniques. It is found that the thermal stability of Au atoms at the Si(111) surface does not change due to deposition of both Ag and Cu atoms and subsequent annealing. On the other hand, it is found that the thermal stability of Ag at the Si(111) surface decreases due to coexistence of Au atoms on the post annealing but is not influenced by coexistence of Cu atoms. Moreover, it is shown that Cu atoms deposited on the Si(111)-√3 × √3-Ag and √3 × √3-Au surfaces become very unstable on the post annealing.

The interaction of Ni adsorbate with the AgSi(111) surface

Abstract The change of atomic structures and compositional ratios by isochronal annealing of Ag–Ni binary adsorbates at the Si(111) surface has been studied by means of LEED-AES-RBS techniques at temperatures from 200°C to 700°C. It is found that the surface coverage of Ni deposited on the Si(111)−√3 × √3−Ag surface decreases to 0 ML on annealing at 250°C for 15 min, while the Ag surface coverage is not changed by Ni deposition and subsequent annealing up to 350°C. It is also found on annealing at 350°C that Ag on the Si(111)−√19 × √19−Ni surface produces the √3 × √3 LEED pattern and the Ni coverage by AES decreases to 0 ML. These results indicate that Ag and Ni atoms are not bound to each other on the Si(111) surface, and that Ag atoms preferentially occupy the surface with forming the √3 × √3−Ag structure and Ni atoms dissolve into the Si bulk. Moreover, it is found that the room temperature deposition of Ni onto the Si(111)−3 × 1−Ag surface produces a mixed structure of √3 × √3 and 3 × 1, which is changed into the single √3 × √3 structure by subsequent annealing at 200°C. It indicates that Ag atoms move from the 3 × 1−Ag site into the √3 × √3−Ag site due to the co-existence of Ni adsorbates.

Ar+ ion impact desorption of Cu and Ag from the Si(111)-quasi-5 × 5-Cu and CuSi(111)-√3 × √3-Ag surfaces

Abstract The concentration changes of Cu and Ag coverages at the Cu Si (111) -√3 × √3- Ag and Si(111)-quasi-5 × 5-Cu surfaces by 5 keV Ar+ ion bombardment have been measured by means of AES and Rutherford backscattering (RBS) techniques in order to determine the cross-sections for their desorption and recoil-implantation. We have evaluated the potential barrier heights for desorption and recoil-implantation from the obtained cross-sections. The potential barrier height for desorption of Ag adsorbates is in accordance with the sublimation energy of Ag metal, and the potential barrier height for recoil-implantation of Ag is consistent with that for the Si(111)-√3 × √3-Ag surface. Together with the observed √3 × √3 LEED pattern for the Cu Si (111) -√3 × √3- Ag surface, these results suggest that Ag atoms are accommodated at the top-most surface layer as the Si(111)-√3 × √3-Ag surface. On the other hand, the cross-section for desorption of Cu is smaller than that of Ag, which leads to rather high potential barrier height, and the cross-section for recoil-implantation is considerably larger than that of Ag. For these results, we propose a hypothesis that Cu atoms are buried and located at an interstitial site in the surface layer of the Cu Si (111) -√3 × √3- Ag surface. It is also found that the Si(111)-quasi-5 × 5-Cu structure is easily destroyed by Ar+ ion bombardment.

Thermal behavior of Cu films on the Si( 111) surface in the monolayer regime

Abstract The isothermal annealing of Cu films deposited on the Si(111)−7 × 7 surface in the ML regime has been studied at temperatures below 520°C by means of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and Rutherford backscattering spectrometry (RBS). For the case of 2.0–3.1 ML in thickness ( 1 ML = 7.84 × 10 14 atoms cm 2 ), it is found at temperatures between 200 and 460°C that 1 ML of Cu atoms constitute a quasi-5 × 5 structure at the top-most surface and the excess Cu atoms diffuse slowly into the bulk when the surface has never been irradiated with 1.5 MeV 4 He + ions. It is also found that the Cu atoms diffuse rapidly when the surface is irradiated by 1.5 MeV 4 He + ions for RBS measurements and is successively annealed. It is determined that the activation energy for diffusion of Cu atoms in the Si crystal is 1.2 ± 0.4 eV at temperatures from 415 to 460°C. For the case of 0.8–1.0 ML in thickness, the LEED pattern shows the quasi-5 × 5 structure and the Cu coverages decrease exponentially upon isothermal annealing at temperatures from 465 to 520°C. It is determined from the decay analysis that the activation energy for Cu atoms to be dissolved from the quasi-5 × 5- Cu Si (111) surface into the Si crystal is 2.6 ± 0.3 eV.

The interaction of Cu atoms with the Si(111)-3 × 3-Ag surface

The room-temperature deposition of Cu onto the Si(111)-3 × 3-Ag surface has been studied by means of LEED-AES-RBS techniques. As the Cu deposition increases, the 3 × 3 LEED pattern is changed into a 1 × 1 pattern at a thickness of about 4 ML, which disappears at about 10 ML. An ordered structure of (1 × 1)R30° appears at 18 ML. In AES spectra, the intensity of the Si signal decreases with increasing Cu deposition, while the Ag signal does not change at all after an initial small decrease at 1 ML. The RBS-channeling measurement shows that Cu atoms are situated at an interstitial site at a Cu coverage of 1.5 ML. The Si surface peak in the RBS-channeling spectra increases linearly with the Cu coverage in a proportion of CuSi = 31. These results indicate that Cu atoms deposited onto the Si(111)-3 × 3-Ag surface at the initial stage up to 4 ML are buried at an interstitial site in subsurface layers of the Si substrate and that a film of Cu3Si is formed at higher coverages.

Dissolution and segregation of monolayer Cu, Ni and Co atoms on the Si(111)-3×3-Ag surface induced by thermal annealing

Abstract Concentration changes of monolayer Cu, Ni, and Co atoms on the Si (111)- 3 × 3 - Ag surface by isochronal annealing at temperatures from 150 to 700°C have been studied by means of LEED-AES-RBS techniques. It is shown that Cu atoms on the Si (111)- 3 × 3 - Ag surface dissolve into the Si bulk at a temperature of 250°C, and segregate back to the surface when Ag atoms decay from the surface on annealing at temperatures higher than 400°C. It is also shown that Ni atoms, which have once dissolved into the bulk, segregate back to the surface on annealing above 400°C. In the case of the Co Si (111)- 3 × 3 - Ag surface, Co atoms preferentially dissolve into the bulk, a situation similar to that of Cu and Ni atoms on the Si (111)- 3 × 3 - Ag surface; however, no segregation of Co atoms has been observed upon annealing at higher temperatures. These results are discussed in terms of the heats of mixing between the Ag atoms and the co-adsorbates and also between the co-adsorbates and the Si substrate.

The change of coverage and structure of the Si(111)-√3 × √3-Ag surface by isothermal annealing

Abstract The thermal stability of the Si(111)-√3 × √3-Ag surface has been studied by means of low energy electron diffraction, Auger electron spectroscopy, and Rutherford backscattering spectrometry techniques. The initial coverages of Ag, which was deposited on the Si(111)-7 × 7 surface at room temperature, are 0.8–1.0 ML ( 1 ML = 7.84 × 10 14 atoms cm 2 ). It is found that on isothermal annealing the coverage of Ag decreases exponentially with increasing time when only the √3 × √3 spots continue to be observed and that the Ag coverage shifts to another slower exponential function at about 0.5 ML, at which the 3 × 1 spots begin to appear between the √3 × √3 spots. The activation energies for Ag atoms to decay from the Si(111)-√3 × √3-Ag and √3 × √3 + 3 × 1-Ag surfaces are determined from the two exponential decay curves to be 0.89 ± 0.21 and 2.6 ± 0.3 eV, respectively. The experimental values are compared with those obtained by other authors.

An in situ RBS system for measuring nuclides adsorbed at the liquid-solid interface

Abstract An in situ RBS system has been developed for measuring heavier nuclides adsorbed at the inner surface of a thin lighter window specimen of a liquid container in order to determine the rate constants for their sorption and release at the liquid-solid interface. The tolerance of a thin silicon window of the sample assembly, in which Xe gas of one atmosphere is enclosed, against the bombardment of the probing ion beam has been tested. A desorption behaviour of a lead layer adsorbed at the SiO2 surface layer of the silicon window into deionized water has been measured as a preliminary experiment.

The change of atomic structures and compositional ratios by thermal annealing of 2D Ag-Cu binary adsorbates on the Si(111) surface

The change of atomic structures and compositional ratios by isochronal annealing of Ag-Cu binary adsorbates at the Si(111) surface has been studied by means of LEED-AES-RBS techniques at temperature from 200 to 450°C. It is found that the Cu/Si(111)- 3 × 3 3-Ag surfaces show the 3 × 3 structure by annealing at 200°C and that the Cu coverage by AES decreases to OML, while the Ag coverage by AES shows almost no change with increasing the temperature up to 350°C. On the other hand, the Ag/Si(111)-quasi5×5-Cu surfaces show a mixed structure of 3 × 3 +quasi5×5 by annealing at 200°C and that the Cu coverage by AES decreases to (1-ϑ Ag 0 ) ML, where ϑ Ag 0 is the initial Ag coverage, while the Ag coverage by AES also shows no change at all. Moreover, it is found that Cu atoms, which have diffused into the bulk from the surface owing to occupation of Ag by the lower temperature annealing, segregate back to the topmost surface when the Ag atoms decay from the surface by the post higher temperature annealing. From these results, it is concluded that Ag and Cu atoms do not interact with each other on Si(111) surfaces.

In‐situ RBS Studies on Dissolution of Pb Atoms from the SiO2 Surface into Water Solutions

An in‐situ RBS system has been developed to study the dissolution of Pb layers deposited physically on the SiO2 surface of Si(100) crystal into water solutions with different pH values. It is found that Pb atoms are not dissolved into alkaline water, but into acid water, and that the dissolution in the latter case is the zero‐th order reaction kinetics and the rate constant is 0.67×1013 atoms cm−2s−1, which corresponds to 1.04×10−11 mol. cm−2s−1. The dissolution mechanism is discussed based on the experimental results.